Fun calculator
Birthday Paradox Calculator
Estimate the chance that at least two people in a group share a birthday.
FormulaP(match) = 1 - product((365 - i) / 365)
Formula
Birthday match probability
P(match) = 1 - product((365 - i) / 365)
A group of 23 people has about a 50% chance of a shared birthday.
FAQ6 common questions for this calculator.
FAQs
Does this include leap years?+
No. It uses 365 equally likely birthdays for a simple estimate.
Why is the probability so high?+
Every pair of people creates a possible match, so comparisons grow quickly.
How does the Birthday Paradox Calculator calculate the result?+
For the Birthday Paradox Calculator, it uses the Birthday match probability: P(match) = 1 - product((365 - i) / 365). A group of 23 people has about a 50% chance of a shared birthday.
What information do I need to use the Birthday Paradox Calculator?+
Enter the values requested by the form, keep units consistent, and use the displayed assumptions to decide whether the Birthday Paradox Calculator matches your scenario.
How accurate is the Birthday Paradox Calculator?+
Birthday Paradox Calculator is accurate for the values and assumptions entered on this page. It uses the Birthday match probability. Recheck the inputs before relying on the estimate.
What should I check before using the Birthday Paradox Calculator result?+
For the Birthday Paradox Calculator, check that the units, inputs, and assumptions match your situation. Change one input at a time to understand which values have the largest effect on the result.
RelatedNearby fun calculators for the next step after Birthday Paradox Calculator.
Fun guide
How to use the Birthday Paradox Calculator
Estimate the chance that at least two people in a group share a birthday. The page also explains the birthday match probability and shows a practical example: A group of 23 people has about a 50% chance of a shared birthday.
- 1
Enter your details
Enter the values for the birthday paradox calculator scenario you want to check.
- 2
Check the calculation
Review the result alongside the birthday match probability: P(match) = 1 - product((365 - i) / 365).
- 3
Compare scenarios
Change one or more inputs to see how they affect the birthday Paradox Calculator result before you use the estimate.